1. Field of the Invention
The invention relates to a pressure swing adsorption process for the separation of gases. More particularly, it relates to a improved rapid pressure swing adsorption process.
2. Description of the Prior Art
The rapid pressure swing adsorption process has been developed in order to extend the advantages of the well-known pressure swing adsorption technology to applications such as the production of breathing oxygen from air. Such oxygen is used for high altitude breathing in aircraft and by persons suffering from respiratory ailments. The rapid pressure swing adsorption, i.e., RPSA, process overcomes the disadvantages inherent in conventional pressure swing adsorption techniques when applied to such applications, e.g., the low adsorbent productivity for supplying breathing oxygen to an individual user.
The RPSA process, which has been described with respect to various particular embodiments in the Earls ar al. patent, U.S. Pat. No. 4,194,891, and in the Jones et al. patent, U.S. Pat. No. 4,194,892, broadly involves the cyclic process of feed and adsorption at higher pressure, exhaust and repressurization carried out on a very rapid basis, i.e., in less than about 30 seconds, in a manner not feasible in the conventional PSA processing cycles used to produce oxygen or other product gases on a commercial scale. Desirable features of the RPSA process are the compactness and light weight of the systems used to carry out the process, these features pertaining without sacrifice in the high oxygen or other product gas enrichment obtainable or in the advantageously high productivity of the RPSA process and system.
On the basis of the general knowledge of the art with respect to pressure swing adsorption technology, operation of the RPSA process at higher feed gas pressure--adsorption levels would be expected to increase the incremental loading of the more readily adsorbable component, e.g., nitrogen in air separation applications, thereby improving the productivity of the RPSA process. In this regard, productivity has been defined as the pounds of pure oxygen, or other desired less readily adsorbable component, produced in the product gas per pound of adsorbent used per unit of time. The RPSA process has heretofore been successfully applied for air separation at relatively low pressure feed gas pressures, that is at pressures of less than about 25 psig, e.g., at pressures generally on the order of about 20 psig. Attempts to operate the RPSA process, of proven design at such lower pressures, at higher feed gas pressure--adsorption levels have not proven entirely satisfactory.
Problems encountered upon efforts to extend the RPSA process to higher feed gas pressure levels, e.g., from 20 psig to about 40 psig, have included considerable difficulty in controlling the stability of product purity as the demand rate is increased in high product enrichment applications, as when 90-95% oxygen is required. In circumstances where product enrichment is critical, as where, for example, 94.5% oxygen may be specified, it has been found necessary to significantly reduce the productivity of the RPSA system in order to meet the product purity requirements.
In addition, the efficiency of the product separation from the feed gas mixture has decreased as the feed gas pressure has been increased to the higher levels indicated above. Such a decrease in oxygen recovery will result in an increase in the amount of the feed gas mixture that must be processed by the RPSA system if the overall product demand rate and product purity are to be maintained. This results in an increase in pumping power costs and in the size of the feed compressors employed in the RPSA process.
Such problems of product purity and recovery are aggravated by the strict limitations imposed on the total cycle time for each bed in RPSA processing operations. As the feed gas mixture is continuously passed to the system for feeding to each bed in turn, in the course of continuous, cyclic RPSA processing operations, the total cycle time for each adsorbent bed to complete its cycle of adsorption, exhaust, repressurization, and any delay periods desirably included therein, is necessarily very short, particularly in applications in which a limited number of adsorbent beds are emloyed for high productivity, high purity applications. As at least one adsorbent bed will always be accepting the pressurized feed gas mixture to be separated in continuous processing operations as heretofore developed, the time available for the exhausting of the more readily adsorbable gas component from the bed becomes extremely restricted.
Confronted with such problems, it is not feasible to simply lengthen the adsorption beds in various high pressure RPSA applications for which the size and weight of the system is critical to its success. In on-board oxygen enrichment systems for military aircraft, as an example, it is not feasible to employ longer, i.e., heavier, adsorbent beds to overcome the disadvantages associated with higher pressure operations. The alternative of decreasing the amount of feed gas entering each bed during the feed portion of the cycle, by decreasing the feed time for each bed, is also disadvantageous as a decrease in feed time in a continuous process cycle is achieved only by a decrease in the total cycle time for each bed. This creates additional processing and mechanical problems. Thus, the desorption and purge time of the exhaust step must also be shortened, which adversely affects the loading capacity of the adsorbent. In addition, the rapid speed of each cycle under such conditions tends to impose problems in the valves for the process that cannot be tolerated in practical, commercial RPSA operations. There exists, therefore, a genuine need in the art for improvements in the RPSA technology to enable higher productivity to be achieved for a given product purity without increasing the size of the adsorption system.
It is an object of the invention, therefore, to provide an improved RPSA process.
It is another object of the invention to provide an RPSA process capable of enhancing productivity for a given product gas purity without an increase in the size of the adsorption system.
It is a further object of the invention to provide an RPSA process having improved performance at high pressures above about 25 psig.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.